1. Field of the Invention
The present invention generally relates to power supply units, and more particularly, to a power supply unit for a DC-DC conversion.
2. Description of the Related Art
FIG. 1 is a schematic circuit diagram of a conventional power supply unit. A power supply unit 1 is an AC adapter converting an AC input supplied from an AC power supply 2 connected thereto into a DC voltage, which is supplied to a load 3.
The power supply unit 1 includes a fuse 4, a rectifier circuit 5 on a primary side, a capacitor C1, a transformer 6, a transistor Q1, a control circuit 7, a rectifier circuit 8 on a secondary side, and an output voltage detecting circuit 9.
The fuse 4 is blown to cut off the supply of current when an overcurrent flows from the AC power supply 2 into the power supply unit 1. The rectifier circuit 5 on the primary side is a bridge rectifier circuit including four diodes, and rectifies an alternating current supplied from the AC power supply 2. The current rectified by the rectifier circuit 5 on the primary side is supplied to one end of a primary winding L1 of the transformer 6. The transistor Q1 is connected to another end of the primary winding L1 thereof.
The transistor Q1 is a field-effect transistor, which includes a drain connected to the other end of the primary winding L1, a source connected to a wiring line 16 on the negative-potential side of a circuit on the primary side, and a gate connected to the control circuit 7. The control circuit 7 controls a DC output voltage by switching ON/OFF the transistor Q1 by controlling the gate potential of the transistor Q1.
The control circuit 7 supplies the gate of the transistor Q1 with a switching pulse having a predetermined frequency, in accordance with which the transistor Q1 is switched ON/OFF. A pulsed current flows through the primary winding L1 by the ON/OFF switching operation of the transistor Q1.
The transformer 6 includes the primary winding L1 and a secondary winding L2, and generates a voltage in the secondary winding L2 in accordance with the current flowing through the primary winding L1. The above generated voltage depends on the ratio of the number of turns on the primary winding L1 to that of turns on the secondary winding L2.
The voltage generated in the secondary winding L2 is supplied to the rectifier circuit 8 on the secondary side, which includes a diode D1 and a capacitor C2. The diode D1 performs a half-wave rectification on an alternating current generated in the secondary winding L2, and the capacitor C2 absorbs the pulsation of the rectified voltage. The voltage rectified by the rectifier circuit 8 on the secondary side is supplied to the load 3 as an output voltage.
The output voltage detecting circuit 9, which includes resistors R1 through R4, a photo-coupler 10, and a shunt regulator 11, detects the output voltage supplied to the load 3. The resistors R1 through R3 divide the output voltage, and a divided voltage is applied to the input terminal of the shunt regulator 11.
FIG. 2 is a circuit diagram of the shunt regulator 11. The shunt regulator 11 compares a voltage Cont of the connecting point of the resistor R2 with the resistor R3 and a reference voltage Vref generated inside the shunt regulator 11, and outputs a voltage based on the result of the comparison. The shunt regulator 11 includes a Zener diode 12, a differential amplifier 13, and an NPN transistor 14. The Zener diode 12 generates the reference voltage Vref. The differential amplifier 13 compares the reference voltage Vref generated by the Zener diode 12 and the voltage Cont of the connecting point of the resistor R2 with the resistor R3, and supplies a current to the base of the NPN transistor 14 in accordance with the voltage difference therebetween. The NPN transistor 14, which is controlled by an output signal of the differential amplifier 13, includes a collector connected to the photo-coupler 10 and an emitter connected to the lower potential side of the load 3.
The differential amplifier 13 increases/decreases the base potential of the NPN transistor 14 as the potential of the connecting point of the resistor R2 with the resistor R3 increases/decreases.
The shunt regulator 11 draws more/less current from the photo-coupler 10 as the potential of the connecting point of the resistor R2 with the resistor R3 increases/decreases, that is, the load 3 consumes less/more current.
The photo-coupler 10 includes a light-emitting diode D2 and a phototransistor Q2 disposed to face the light-emitting diode D2. The light-emitting diode D2 is connected to the shunt regulator 11. The light-emitting diode D2 emits more/less amount of light as the shunt regulator 11 draws more/less current.
The phototransistor Q2, which includes a collector connected to a wiring line 15 on the positive-potential side of the circuit on the primary side, and an emitter connected to the control circuit 7, controls a current flowing from the emitter in accordance with the amount of light emitted from the light-emitting diode D2.
The control circuit 7, which is connected to the emitter of the phototransistor Q2 of the photo-coupler 10, supplies the switching pulse to the gate of the transistor Q1 in accordance with the current flowing from the emitter of the phototransistor Q2.
The transistor Q1 is switched ON/OFF by the switching pulse supplied from the control circuit 7. The transformer 6 transmits an electric power from the primary winding L1 to the secondary winding L2 in accordance with the ON/OFF switching operation of the transistor Q1.
The control circuit 7 is connected to the wiring line 15 on the positive-potential side of the circuit on the primary side, and thus is constantly supplied with a driving current so as to be in an ever-driven state.
When the current consumed in the load 3 is reduced, the output voltage between outputs terminals Tout1 and Tout2 starts to rise. Then, the potential of the connecting point of the resistor R2 with the resistor R3 also starts to rise, thus resulting in an increase in the current which the shunt regulator 11 draws. In accordance with this increase in the current, the light-emitting diode D2 of the photo-coupler 10 emits more amount of light, and more current flows from the emitter of the phototransistor Q2 into the control circuit 7.
The control circuit 7 then controls the switching pulse supplied to the gate of the transistor Q1 so that the transistor Q1 is ON for a shorter period of time, thus reducing the electric power transmitted from the primary winding L1 of the transformer 6 to the secondary winding L2 thereof. Then, the output voltage between the output terminals Tout1 and Tout2 is controlled to become lower to keep a constant value.
On the other hand, when the current consumed in the load 3 is increased, the output voltage between outputs terminals Tout1 and Tout2 starts to lower. Then, the potential of the connecting point of the resistor R2 with the resistor R3 also starts to lower, thus resulting in a decrease in the current which the shunt regulator 11 draws. In accordance with this decrease in the current, the light-emitting diode D2 of the photo-coupler 10 emits less amount of light, and less current flows from the emitter of the phototransistor Q2 into the control circuit 7.
The control circuit 7 then controls the switching pulse supplied to the gate of the transistor Q1 so that the transistor Q1 is ON for a longer period of time, thus increasing the electric power transmitted from the primary winding L1 of the transformer 6 to the secondary winding L2 thereof. Then, the output voltage between the output terminals Tout1 and Tout2 is controlled to become higher to keep a constant value.
However, according to the conventional power supply unit 1, there exists a problem that the control circuit 7 is driven to unnecessarily consume a current, or electric power even if the load 3 is disconnected. This is because the control circuit 7 is driven by a DC voltage which is obtained by rectifying, via the rectifier circuit 5 on the primary side and the capacitor C1, the AC input supplied from the AC power supply 2 to the side of the primary winding L1 of the transformer 6.
It is a general object of the present invention to provide a power supply unit in which the above disadvantage is eliminated.
A more specific object of the present invention is to provide a power supply unit which can reduce the power consumption.
The above objects of the present invention are achieved by a power supply unit including a transformer converting an input voltage into a predetermined output voltage, a transistor controlling a current flowing through a primary winding of the transformer, an output voltage detecting circuit detecting an output voltage of the power supply unit and outputting a detection signal corresponding to a level of the output voltage, a control circuit controlling the output voltage by switching on and off the transistor in accordance with the detection signal, and an operation control circuit controlling an operation of the control circuit in accordance with the output voltage.
According to the above power supply unit of the present invention, the supply of the main driving voltage to the control circuit is cut off, and the control circuit is controlled to only intermittently operate by the operation control circuit when the load is disconnected from the power supply unit, and the output voltage falls below a predetermined value. Therefore, the power consumption thereof can be reduced.
The above objects of the present invention are also achieved by a power supply unit including a transformer, a first control circuit controlling switching of a current flowing through a primary winding of the transformer, and a second control circuit causing the first control circuit to be intermittently enabled when no load is connected to a secondary winding of the transformer.
The above objects of the present invention are also achieved by a power supply unit including a transformer, a first control circuit controlling switching of a current flowing through a primary winding of the transformer, and a power supply system supplying power to the first control circuit so that the power is constantly supplied thereto when a load is connected to a secondary winding of the transformer and is intermittently supplied when no load is connected.